Bandpass filter

ABSTRACT

The bandpass filter has a comparatively large pass bandwidth, with, at the same time, comparatively steep edges up to the stop band and low attenuation in the passband. The bandpass filter contains three parallel LC elements, one of which is connected between the bandpass filter input and the bandpass filter output. The other two parallel LC elements each have one of their connections coupled to a fixed reference-ground potential.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of copending International ApplicationPCT/DE98/01846, filed Jul. 3, 1998, which designated the United States.

BACKGROUND OF THE INVENTION Field Of The Invention

The invention lies in the electronics field. More specifically, theinvention relates to a bandpass filter, particularly for use in circuitsusing RF technology.

In many circuit configurations using RF technology, for example incircuit configurations for contiguous division of a relatively largefrequency range (e.g. the TV frequency range) into a plurality ofsmaller frequency bands, bandpass filters are required which have acomparatively large pass bandwidth, with, at the same time,comparatively steep edges up to the stop band and low attenuation in thepassband.

French patent document FR 952403 discloses a bandpass filter circuit.There, a capacitor is connected in series with a parallel LC element.The filter four-terminal circuit has an additional parallel LC elementat the output.

U.S. Pat. No. 5,483,209 discloses a circuit for changing over betweendifferent reception bands with variable attenuation control. The circuithas a plurality of bandpass filters tuned to the different inputreception frequencies. In addition, the circuit has a plurality ofvariable attenuation diodes whose impedances are controlled by means ofan AGC. A plurality of switching diodes are used to select the bandpassfilter tuned-to the input signal.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a bandpassfilter, which overcomes the above-mentioned disadvantages of theheretofore-known devices and methods of this general type and whichsatisfies the aforementioned requirements.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a bandpass filter, comprising:

-   -   a bandpass filter input and a bandpass filter output;    -   a series circuit connected between the bandpass filter input and        the bandpass filter output, the series circuit being formed of a        first capacitor, a first parallel LC element connected to the        first capacitor, a second capacitor connected to the first        parallel LC element, and an inductor connected to the second        capacitor;    -   a second parallel LC element having a first connection connected        to a node between the first parallel LC element and the second        capacitor and a second connection coupled to a fixed        reference-ground potential via a third capacitor; and    -   a third parallel LC element having a first connection connected        to a node between the second capacitor and the inductor and a        second connection coupled to the fixed reference-ground        potential, either directly or via a fourth capacitor connected        between the second connection of the third parallel LC element        and the fixed reference-ground potential.

In accordance with an additional feature of the invention, a fifthcapacitor is connected to a node between the second capacitor and theinductor and to the fixed reference-ground potential.

With the above and other objects in view there is also provided, inaccordance with the invention, a circuit configuration, comprising:

-   -   an AC voltage input terminal and an AC voltage output terminal;    -   a plurality of frequency domain filter paths defined between the        AC voltage input terminal and the AC voltage output terminal,        and connected in parallel between a common first node and a        common second node both coupled to a DC voltage connection;    -   each of the frequency domain filter paths containing at least        one of the above-outlined bandpass filters connected in series        with a first diode and a second diode connected in opposite        forward direction from the first diode;    -   each of the frequency domain filter paths containing a switching        unit for switching the first and the second diode in the        frequency domain filter path during an operation of the circuit        configuration for turning a respective one of the bandpass        filters;    -   a third diode having a first terminal connected to the first        node and a fourth diode having a first terminal connected to the        second node of the frequency domain filter paths, such that a        respective cathode of the third diode and of the fourth diode is        connected to anodes of the first diodes and the second diodes,        respectively;    -   a load-dependent DC voltage source having a first connection and        a second connection; and    -   a third diode and the fourth diode each having a second terminal        respectively connected to the first connection and the second        connection of the load-dependent DC voltage source.

In accordance with another feature of the invention, the first, second,third, and fourth diodes are PIN diodes.

In accordance with a further feature of the invention, each of theswitching units includes:

-   -   a first electrical resistor and a second electrical resistor        respectively connected, via a first terminal thereof, to the        input and to the output of an associated one of the frequency        domain filters, and to one another via a second terminal        thereof;    -   an on/off switch having a first terminal connected between the        first electrical resistor and the second electrical resistor,        and a second terminal connected to a fixed reference-ground        potential; and    -   a capacitor having a first terminal connected between the first        electrical resistor and the second electrical resistor, and a        second terminal connected to the fixed reference-ground        potential.

In further summary, in the bandpass filter according to the invention:

-   -   a) a series circuit, comprising a first capacitor, a first        parallel LC element, a second capacitor and an inductor, is        connected between a bandpass filter input and a bandpass filter        output, the individual elements in the series circuit being        connected one after the other in the aforementioned sequence;    -   b) a second parallel LC element, whose second connection is        coupled to a fixed reference-ground potential via a third        capacitor, is connected in a connection line between the first        parallel LC element and the second capacitor; and    -   c) a third parallel LC element, whose second.connection is        coupled to the fixed reference-ground potential directly or via        a fourth capacitor, is connected in a connection line between        the second capacitor and the inductor.

Optionally, the connection line between the second capacitor and theinductor is coupled to the fixed reference-ground potential (e.g.ground) via a fifth capacitor.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a bandpass filter, it is nevertheless not intended to be limited tothe details shown, since various modifications and structural changesmay be made therein without departing from the spirit of the inventionand within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in,connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit schematic of an exemplary embodiment of the filteraccording to the invention; and

FIG. 2 is a schematic diagram of a circuit configuration with bandpassfilters in accordance with the exemplary embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is seen a bandpass filter with aseries circuit formed with a first capacitor C1, a first parallel LCelement L_(p) 1/C_(p) 1, a second capacitor C2, and an inductor L. Theseries circuit is connected between a bandpass filter input IN and abandpass filter output OUT. The circuit elements are connected inseries, one after the other, in the aforementioned sequence. A firstconnection of a second parallel LC element L_(p) 2/C_(p) 2, whose secondconnection is coupled to a fixed reference-ground potential P via athird capacitor C3, is connected between the first parallel LC elementL_(p) 1/C_(p) 1 and the second capacitor C2. A first connection of athird parallel LC element L_(p) 3/C_(p) 3, whose second connection iscoupled to the fixed reference-ground potential P directly or via afourth capacitor C4, is connected between the second capacitor C2 andthe inductor L.

An optional fifth capacitor C5 is illustrated in dashes. The capacitorC5 may be provided to connect the fixed reference-ground potential P tothe node between the second capacitor C2 and the inductor L.

Optionally, a series circuit for the fixed reference-ground potential Pcan also be connected between the second capacitor C2 and the inducer L.

While the drawings illustrate, and the above description refers to,inductors L, L_(p) 1, L_(p) 2, L_(p) 3, it is equally possible to usestriplines as the inductances.

Reference will now be had to the circuit configuration shown in FIG. 2,where a parallel circuit, comprising a plurality of frequency domainfilter paths FZ1, FZ2, . . . , FZi, is connected between an AC voltageinput terminal IN_(RF) and an AC voltage output terminal OUT_(RF). Eachof the frequency domain filter paths has a bandpass filter F1, F2, . . ., Fi in accordance with the exemplary embodiment shown in FIG. 1.

A sixth capacitor C6 is connected between the AC voltage input terminalIN_(RF) and a first node K1 of the parallel circuit, and a seventhcapacitor C7 is connected between a second node K2 of the parallelcircuit and the AC voltage output terminal OUT_(RF). These twocapacitors C6 and C7 are essentially used for DC voltage decoupling ofthe AC voltage connections IN_(RF) and OUT_(RF).

The nodes K1 and K2 each have a regulating voltage connection E_(R)coupled to them via the two inductor elements Dr1, Dr2, which is used tosupply the nodes K1 and K2 with a direct current during operation.Instead of the inductor elements Dr1 and Dr2, suitable nonreactiveresistors can also be used.

In the exemplary embodiment, the regulating voltage connection E_(R) isconnected to the emitter of a pnp transistor T whose collector isconnected to the operating voltage input E_(B) and whose base isconnected to a control voltage connection U_(AGC) via an electricalresistor RV.

Each frequency domain filter path FZ1, FZ2, . . . , FZi comprises abandpass filter F1, F2, . . . , Fi connected between two diodes D11,D21; D12, D22; . . . ; D1i, D2i, preferably PIN diodes, in series withthe latter. The two PIN diodes are connected with oppositely orientedforward directions.

Each bandpass filter F1, F2, . . . , Fi has a DC supply, comprising afirst resistor R11, R12, . . . , R1i and a second resistor R21, R22, . .. , R2i, connected to it between the two associated diodes D11, D21;D12, D22; . . . ; D1i, D2i. The first resistor R11, R12, . . . , R1i andthe second resistor R21, R22, . . . , R2i are respectively connected, onthe one hand, to the input EF1, EF2, . . . , EFi and to the output AF1,AF2, . . . , AFi of the associated bandpass filter F1, F2, . . . , Fiand, on the other hand, to one another. FIG. 2 indicates switching unitsSE1, SE2, . . . , SEi in dash-dotted boxes which each have a firstelectrical resistor R11, R12, . . . , R1i and a second electricalresistor R21, R22, . . . , R2i which are respectively connected betweenthe input and the output of the associated frequency domain filter F1,F2, . . . , Fi and to one another. The respective connection linebetween the two resistors R11, R21; R12, R22; . . . ; R1i, R2i isblocked off to alternating current by means of a capacitor CF1, CF2, . .. , CFi in each case and is connected to a first switch terminal of an“on/off” switch S1, S2, . . . , Si (preferably an electronic switch,e.g. an open-collector switching output of an integrated circuit) ineach case, whose second switch connection is connected to a fixedreference-ground potential.

Connected in parallel with the frequency domain filter paths FZ1, FZ2, .. . , FZi is a series circuit comprising a third diode D3, a firstresistor R1, a second resistor R2 and a fourth diode D4. The two diodesD3 and D4 are connected with oppositely oriented forward directions andare preferably likewise PIN diodes. In addition, the two resistors R1and R2 are connected, on the one hand, to the diodes D3 and D4,respectively, and, on the other hand, to a center tap M1 of a voltagedivider comprising a third resistor R3 and a fourth resistor R4.

The voltage divider is, on the one hand, connected to an operatingvoltage connection E_(B), which is connected to the same voltage sourceas the regulating voltage connection E_(R), for example, and, on theother hand, to the fixed reference-ground potential P. The firstresistor R1 and the second resistor R2 and the voltage divider whichinclude the third resistor R3 and the fourth resistor R4 define aload-dependent DC-voltage source. The end of the first resistor R1 thatis connected to the third diode D3 defines a first connection of theload-dependent DC voltage source and the end of the second resistor R2that is connected to the fourth diode D4 defines a second connection ofthe load-dependent DC voltage source.

An eighth capacitor C8 and a ninth capacitor C9, whose secondconnections are connected to the fixed reference-ground potential P, areconnected between the third diode D3 and the first resistor R1 andbetween the fourth diode D4 and the second resistor R2, respectively.These capacitors C8, C9 serve to derive the AC voltage for the fixedreference-ground potential P (e.g. ground).

Changing over between the individual frequency domain filter paths FZ1,FZ2, . . . , FZi is performed by means of the diode pairs D11, D21; D22;. . . ; D1i, D2i, which are selectively switched on by the switches S1,S2, . . . , Si.

Advantageously, the respectively active diode pair D11, D21; D12, D22; .. . ; D1i, D2i is in this case simultaneously used as a series elementof a regulatable π-attenuation element whose parallel elements are thethird and the fourth diode D3, D4. In the high state, the latter havezero current (high resistance). If the regulating voltage U_(R) isreduced, current flows through them and they become less resistive,whereas the current through the respective active diode pair D11, D21;D12, D22; . . . ; D1i, D2i in the frequency domain filter path FZ1, FZ2,. . . , FZi which is operating falls with the regulating voltage, whichmeans that these diodes become more resistive.

1. A circuit configuration, comprising: an AC voltage input terminal andan AC voltage output terminal; a plurality of frequency domain filterpaths defined between said AC voltage input terminal and said AC voltageoutput terminal, and connected in parallel between a common first nodeand a common second node both coupled to a DC voltage connection; eachof said frequency domain filter paths containing at least one bandpassfilter connected in series with a first diode and a second diodeconnected in opposite forward direction from said first diode; said atleast one bandpass filter including: a bandpass filter input and abandpass filter output; a series circuit connected between said bandpassfilter input and said bandpass filter output, said series circuit beingformed of a first capacitor, a first parallel LC element connected tosaid first capacitor, a second capacitor connected to said firstparallel LC element, and an inductor connected to said second capacitor;a second parallel LC element having a first connection connected to anode between said first parallel LC element and said second capacitorand a second connection coupled to a fixed reference-ground potentialvia a third capacitor; and a third parallel LC element having a firstconnection connected to a node between said second capacitor and saidinductor and a second connection coupled to the fixed reference-groundpotential; each of said frequency domain filter paths containing aswitching unit for switching said first and said second diode in saidfrequency domain filter path; a third diode having a first terminalconnected to said first node and a fourth diode having a first terminalconnected to said second node of said frequency domain filter paths,such that a respective cathode of said third diode and of said fourthdiode is connected to anodes of said first diodes and said seconddiodes, respectively; a load-dependent DC voltage source having a firstconnection and a second connection; and said third diode and said fourthdiode each having a second terminal respectively connected to said firstconnection and said second connection of said load-dependent DC voltagesource.
 2. The circuit configuration according to claim 1, wherein saidsecond connection of said third parallel LC element is directlyconnected to the fixed reference-ground potential.
 3. The circuitconfiguration according to claim 1, which comprises a fourth capacitorconnected between said second connection of said third parallel LCelement and the fixed reference-ground potential.
 4. The circuitconfiguration according to claim 1, wherein said bandpass filter has afurther capacitor having a first terminal connected to a node betweensaid second capacitor and said inductor and a second terminal connectedto the fixed reference-ground potential.
 5. The circuit configurationaccording to claim 1, wherein said first diode, said second diode, saidthird diode, and said fourth diode are PIN diodes.
 6. The circuitconfiguration according to claim 1, wherein each of said switching unitsincludes: a first electrical resistor and a second electrical resistorrespectively connected, via a first terminal thereof, to said input andto said output of an associated one of said frequency domain filters,and to one another via a second terminal thereof; an on/off switchhaving a first terminal connected between said first electrical resistorand said second electrical resistor, and a second terminal connected toa fixed reference-ground potential; and a capacitor having a firstterminal connected between said first electrical resistor and saidsecond electrical resistor, and a second terminal connected to the fixedreference-ground potential.
 7. A circuit configuration, comprising: anAC voltage input terminal and an AC voltage output terminal; a pluralityof frequency domain filter paths defined between said AC voltage inputterminal and said AC voltage output terminal, and connected in parallelbetween a common first node and a common second node both coupled to aDC voltage connection; each of said frequency domain filter pathscontaining at least one bandpass filter connected in series with a firstdiode and a second diode connected in opposite forward direction fromsaid first diode; each of said frequency domain filter paths containinga switching unit for switching said first and said second diode in saidfrequency domain filter path; a third diode having a first terminalconnected to said first node and a fourth diode having a first terminalconnected to said second node of said frequency domain filter paths forconnecting a respective cathode of said third diode and of said fourthdiode to anodes of said first diodes and said second diodes,respectively; a load-dependent DC voltage source having a firstconnection and a second connection; and said third diode and said fourthdiode each having a second terminal respectively connected to said firstconnection and said second connection of said load-dependent DC voltagesource.
 8. The circuit configuration according to claim 7, wherein saidfirst diode, said second diode, said third diode, and said fourth diodeare PIN diodes.
 9. The circuit configuration according to claim 7,wherein each of said switching units includes: a first electricalresistor and a second electrical resistor respectively connected, via afirst terminal thereof, to said input and to said output of anassociated one of said frequency domain filters, and to one another viaa second terminal thereof; an on/off switch having a first terminalconnected between said first electrical resistor and said secondelectrical resistor, and a second terminal connected to a fixedreference-ground potential; and a capacitor having a first terminalconnected between said first electrical resistor and said secondelectrical resistor, and a second terminal connected to the fixedreference-ground potential.